1. Field of the Invention
The invention relates to a shift control system for vehicles and in particular to a shift control system for changing the shift range of an automatic transmission in response to shift operation by the driver.
2. Description of the Related Art
In general, a shift lever to be operated by a driver is provided in vehicles with an automatic transmission. As the driver slides the shift lever, a shift range corresponding to the position of the shift lever is selected, whereby the power transfer state of the geared power transfer mechanism of the automatic transmission is changed accordingly. Shifting the shift lever to the parking position corresponding to the parking range (will be referred to as “P range”) activates a lock mechanism to hold a parking gear formed integrally on the output shaft of the power transfer mechanism, whereby the output shaft is locked and thus the vehicle is maintained in a stopped state.
In recent years, so-called shift-by-wire shift control systems have been used in place of mechanical shift control systems that operate in response to shift lever operations (For example, refer to Japanese patent application publication No. 2002-122236 (JP-A-2002-122236). In shift-by-wire shift control systems, shift operations by the driver are detected by various sensors and switches, and a shift range determined based on their detection signals is established. The P range can be established by operating a parking switch (will be referred to as “P switch”) with one-touch of the driver's finger.
In shift-by-wire shift control systems, however, the shift range may be switched to the P range in response to the P switch being turned on even when the vehicle has not yet completely stopped. In this case, the rotating output shaft is forcibly locked by the lock mechanism, and the resultant reaction force causes the vehicle to rock, making the driver feel uncomfortable, and moreover, vibrations occur due to backlashes at the gears of the power transfer mechanism.